Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell, which includes a magnetic cell core supported by a substrate. The magnetic cell core includes at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region in between. The free regions and fixed regions of STT-MRAM cells may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) with the width of the regions.
The fixed region includes a magnetic material that has a substantially fixed (e.g., a non-switchable) magnetic orientation. The free region, on the other hand, includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell between a “parallel” configuration and an “anti-parallel” configuration. In the parallel configuration, the magnetic orientations of the fixed region and the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively). In the “anti-parallel” configuration the magnetic orientations of the fixed region and the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively).
In the parallel configuration the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements, i.e., the fixed region and free region. This state of low electrical resistance may be defined as a “0” logic state of the MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements, i.e., the regions of magnetic material, e.g., the fixed region and free region. This state of high electrical resistance may be defined as a “1” logic state of the STT-MRAM cell.
Switching of the magnetic orientation of the free region may be accomplished by passing a programming current through the magnetic cell core and the fixed and free regions therein. The fixed region within the magnetic cell core polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current interacts with the free region by exerting a torque on the free region. When the torque of the spin-polarized electron current passing through the core is greater than a critical switching current density (Jc) of the free region, the torque exerted by the spin-polarized electron current is sufficient to switch the direction of the magnetic orientation of the free region. Thus, the programming current can be used to alter the electrical resistance across the magnetic regions. The resulting high or low electrical resistance states across the magnetoresistive elements enables the write and read operations of the conventional MRAM cell. After switching the magnetic orientation of the free region to achieve the one of the parallel configuration and the anti-parallel configuration associated with a desired logic state, the magnetic orientation of the free region is usually desired to be maintained, during a “storage” stage, until the MRAM cell is to be rewritten to a different configuration (i.e., to a different logic state).
A magnetic region's magnetic anisotropy (“MA”) is an indication of the strength of its magnetic orientation and, therefore, an indication of the magnetic material's resistance to alteration of the magnetic orientation. A magnetic material exhibiting a magnetic orientation with a high MA strength may be less prone to alteration of its magnetic orientation out of that orientation than a magnetic material exhibiting a magnetic orientation with a lower MA strength.
The amount of programming current required to switch the free region from the parallel configuration to the anti-parallel configuration is affected by the MA strength of the magnetic regions. A free region with a stronger (i.e., a higher) MA strength may require a greater amount of programming current to switch the magnetic orientation thereof than a free region with a weaker (i.e., a lower) MA strength. However, a free region with a weak MA strength is also often less stable during storage. That is, a free region with a weak MA strength is prone to premature alteration out of its programmed configuration (i.e., the programmed parallel or anti-parallel configuration), particularly when the fixed region of the MRAM cell has a strong MA strength. Therefore, it is often a challenge to form an MRAM cell with a free region and a fixed region having MA strengths that enable switching with minimized programming current without deteriorating the cell's ability to store the programmed logic state without failure (i.e., without premature switching of the magnetic orientation of the free region).